Each year around a thousand people attempt to reach the peak of Mount Everest, which is the tallest mountain on Earth, rising approximately 9 km above sea level. On Mars, the mountain Olympus Mons goes up about 22 km, which more than doubles the height of Mount Everest. Furthermore, Galileo Galilei observed that the Moon also has mountains. What about mountains on other types of astrophysical bodies? How tall are they? Can we climb them?
Charles Horowitz, a nuclear astrophysicist at Indiana University – Bloomington in the Physics Department, and Jorge Morales, his doctoral student, recently published a peer-reviewed paper at the Monthly Notices of the Royal Astronomical Society that addresses these questions for neutron stars.
Neutron stars are the dead remnants of massive stars. A typical neutron star contains half a million times the mass of the Earth within a sphere with a radius of 10 km. This fits comfortably within half of Indianapolis and is approximately twice as tall as Mount Everest. Therefore, these astrophysical wonders are extremely dense. In fact, at their centers, neutron stars can be a few thousand trillion times denser than water. The outermost layer of neutron stars, which spans a kilometer or so, is a frozen solid known as ‘the crust’, which floats over a deep liquid ocean. Several dynamical astrophysical processes (like starquakes, tidal forces, and nuclear reactions) can lead to the formation and support of mountains on the surface of and within the crust.
In their recently published paper, Charles Horowitz and Jorge Morales show that mountains can be as tall as a few centimeters high. Therefore, the tallest possible mountains on the neutron star crust are roughly a million times smaller than Mount Everest and are better thought of as tiny bumps on the surface of or within the crust. This has the remarkable implication that neutron stars are more spherical than the world’s roundest object – a 100 cm radius silicon sphere stored at the Centre for Precision Optics, located in Australia, that is useful to define the kilogram. Also, it might feel disheartening to know that we will not be able to climb a neutron star mountain if we go on to hike on the surface of the neutron star. However, even walking on the surface is impossible, as temperatures can be as high as 2 million degrees Fahrenheit (this will melt the hiker) and gravity is 2 billion times stronger than on the Earth (this will crush the hiker). The good news is that we might be able to observe these tiny bumps, if the neutron star is spinning 400 times a second and is roughly 3,000 light years away or closer (see this blog for further information on how we might observe neutron star mountains with Einstein’s gravitational waves).
Thanks to Prof. Charles J. Horowitz for his helpful suggestions.
Edited by Jonah Wirt and Elizabeth Rosdeitcher